In a large scale integration (LSI), as the degree of integration and the capacity are increased, the circuit size required for a semiconductor device has been gradually narrowing. In the manufacture of the semiconductor device, a pattern is exposed and transferred onto a wafer by a reduced projection exposure apparatus generally called a stepper or scanner to form a circuit, using an original image pattern (meaning a mask or a reticle and hereinafter collectively referred to as a mask) formed with a circuit pattern, whereby the semiconductor device is manufactured.
Enhancement of yield is essential, as the manufacture of LSI requires a large manufacturing cost. Meanwhile, a recent typical logic device is under such a condition that the formation of a pattern with a line width of several ten nm is required. In these circumstances, a pattern defect of a mask and variation of process terms and conditions at the time of exposure and transfer constitute a major cause of reduction in the yield. Along with miniaturization of an LSI pattern dimension formed on a semiconductor wafer, the size of the pattern defect of a mask is also miniaturized. The dimensional accuracy of the mask is enhanced, whereby the fluctuation of the process terms and conditions is to be absorbed, and thus, in the inspection of a mask, an extremely small pattern is required to be detected. As a result, high inspection accuracy is required for an inspection apparatus, which detects a defect of a mask for transfer to be used in the manufacture of LSI.
In the inspection apparatus, light emitted from a light source is applied to a mask, which is to be inspected, through an optical system. The mask is placed on a table, and the irradiated light scans the mask by movement of the table. The light transmitted through or reflected by the mask is imaged on an image sensor through a lens, and an optical image imaged by the image sensor is sent as measurement data to a comparison part. In the comparison part, the measurement data and reference data are compared with each other in accordance with a suitable algorithm. When this data does not coincide with each other, it is determined that there is a defect (see, for example Japanese Patent Laid-Open Publication No. 2008-112178).
In response to the miniaturization of a pattern formed on a mask, realization of high magnification and high numerical aperture is advanced in an inspection optical system used for imaging an optical image of the pattern. Thus, a focus depth as an allowable range of a distance between an optical system and the mask becomes deeper, and a pattern image is blurred simply by slightly changing the distance between the optical system and the mask, so that a defect detection processing is hampered. Because of this, an automatic focus mechanism is used to fix the distance between the optical system and the mask at all times.
Japanese Patent Laid-Open Publication No. 2003-294420 discloses an automatic focus mechanism which a focal position of an inspection optical system is aligned with a surface of a mask. In this automatic focus mechanism, when the mask is irradiated with light from a light source, the light reflected by the mask enters an optical sensor. Subsequently, an electrical signal of the incident light is converted into a digital signal and then input into a height measuring circuit. In the height measuring circuit, a differential signal with respect to an input offset value and a target height are output. The differential signal is input to a Z table driving circuit used for driving a Z table. Then, the Z table drive circuit drives the Z table in accordance with the differential signal. Consequently, the distance between the optical system and the mask can be fixed.
Recently, as a technique for forming a fine pattern, nanoimprint lithography (NIL) has attracted attention. In this technique, a mold (die) having a nanoscale microstructure is pressure applied to a resist on a wafer to form the fine pattern on the resist.
In the nanoimprint technology, to increase productivity, duplicate templates (replica templates) are produced using a master template as an original plate, and the replica templates are mounted and used in different nanoimprint apparatuses. The replica template is required to be produced so as to accurately correspond to the master template. Thus, high inspection accuracy is required when the replica template is inspected.
The replica template has a mesa structure in which the central portion protrudes relative to the outer circumferential portion, and a pattern is formed on the protruding portion (referred to as a mesa portion or a land portion). According to this structure, when the pattern is transferred onto a resist, the occurrence of unnecessary contact between the replica template and the resist can be prevented.
When a pattern defect of the replica template is inspected, a surface of the replica template is scanned with light from a light source. At this time, since there is a step between the mesa portion and the other portions, there is a problem that tracking of a focus control using an automatic focus mechanism cannot be performed. For example, the light is applied to the step or an end of the mesa portion, and when the light diffused by the step or the end of the mesa portion is reflected to enter a height measuring circuit, the light may focus on a position which is not a focal position. Alternatively, when the light passes through the step, although the height of a table is significantly reduced to incorporate the portion into an imaging surface, if the control using a Z table driving circuit cannot be tracked at this time, the distance between the optical system and a surface to be inspected is no longer fixed, so that a pattern image is blurred.
The present invention has been made in consideration of the above points, and provides an inspection method and an inspection apparatus in which a sample having a mesa portion with a pattern can be accurately inspected.
Other challenges and advantages of the present invention are apparent from the following description.